1. Field of the Invention
The present invention relates to a droplet discharge control method, and a liquid discharge apparatus, and more particularly to a droplet discharge control technique which forms a favorable image, drawing, or another shape with consideration for the effect of mutual interference of the droplets as they land.
2. Description of the Related Art
In recent years, inkjet printers have come to be used widely as data output apparatuses for outputting images, documents, or the like. An inkjet printer forms data on recording paper by driving recording elements (nozzles) of a recording head in accordance with data, thereby causing ink to be ejected from the nozzles.
In an inkjet printer, a recording head with a large number of nozzles and a recording medium are relatively moved and a desired image is formed on the recording medium by ejecting ink droplets from the nozzles.
There is a demand for high-speed and high-quality printing in inkjet recording apparatuses, and high-speed printing is achieved by reducing the ink droplet discharge cycle, conveying the recording medium at high speed, and performing other techniques.
In order to print high-quality images, on the other hand, high-quality half-toning and higher resolution is achieved by making the dots that form the image more minute and ensuring higher density. Making the dots smaller is achieved by discharging ink in small amounts, for example, and increasing the density of the dots by forming with greater density the nozzles which eject ink. When the dots are made more highly dense, the spacing between dots that are formed in mutually adjacent positions results in an overlapping formation area thereof.
In a row of dots in which a plurality of dots are formed so as to overlap, when the next droplet lands before the previously landed ink droplet has completed permeation or before the ink droplet has finished curing, the ink droplet that landed afterward is drawn to the previously landed ink droplet, and it is possible that landing interference may occur in which a dot of the target size is not formed in the target dot formation position. Therefore, in order to inhibit such mutual interference (landing interference) between droplets when they land, droplet discharge is controlled so as to wait for the ink that previously landed to permeate and then discharge the ink droplet that will land next. However, it is difficult to achieve high-speed printing with droplet discharge control whereby the system waits for the ink that previously landed to permeate and then discharges the ink droplet that will land next.
The related art is more specifically described below with reference to FIGS. 13A to 15D.
FIGS. 13A and 13B are diagrams that describe the discharge control of the related art.
FIG. 13A shows a row 200 of dots formed on a recording paper using an inkjet recording apparatus. In the row 200 of dots, dots 202, 204, 206, and so forth having the same diameter D are aligned on the same line with the same pitch P between the dots.
In FIG. 13A, the pitch P between the dots is set to be less than the diameter D of the dots (that is to say, D>P), and the row 200 of dots is formed so that the adjacent dots are mutually overlapping.
FIG. 13B shows a line drawing 240 composed of the ink droplets which form the dots in the row 200 of dots shown in FIG. 13A, formed on the recording paper in which the ink droplets are discharged with a discharge cycle (discharge frequency) to an adjacent deposition point before the landed ink droplets completely permeate the recording paper. It should be noted that the key symbol 202′ shown by the broken line 202 shows the intended (theoretical) shape of the line drawing 240 that was to be formed. Also, the discharge sequence of the ink droplets that form the row 200 of dots is the direction indicated by the arrow K (left to right direction in FIG. 13A).
As used herein, the term “dot” commonly refers to a substantially circular shape formed when an ink droplet discharged onto recording paper is fixed on the recording paper, and represents a substantially circular shape formed after the ink droplet has completed permeation. In the present specification, when the shape of the ink droplet breaks down and a shape that is different from a substantially circular shape is formed, or when a shape is formed that differs from a shape formed by the overlapping of a plurality of substantially circular shapes due to phenomena such as the grouping of a plurality of ink droplets, the shape formed by the deposition points is also referred to as a dot.
The present inkjet recording apparatus waits for a droplet discharge frequency at which ink droplets are discharged to an adjacent deposition point in at least one direction selected from the forward, rearward, leftward, and rightward directions before an ink droplet discharged to a certain deposition point has completely permeated the recording paper. For example, droplet discharge is carried out so that the ink that forms the dot 204 adjacent to the dot 202 lands on the recording paper before the ink droplet that forms the dot 202 completes permeation.
In this manner, when the ink droplet that forms a dot (dot 204) adjacent to a dot (dot 202) formed by the ink droplet is discharged before the ink droplet which has landed on the recording paper (ink droplet that forms dot 202, for example) completes permeation, the ink droplet that landed afterward connects with the previously landed ink droplet, and these become a single unit to form an image. This is a phenomenon that occurs when ink droplets are successively (same amounts) discharged, and due to aggregation immediately after the start of discharge the ink droplet that landed afterward is drawn to the previously landed ink droplet.
Originally, a line with a uniform (line width h=D) width should be formed when the ink droplets that form the row 200 of dots are successively discharged, but due to the above-described aggregation, a line drawing 240 in which the line width h=D′ (that is to say, D′>D) is formed immediately after writing is started, as shown in FIG. 13B.
This is substantially equivalent to when a dot 202′ with a diameter D′ is formed by aggregation at the droplet deposition point at which the dot 202 with the intended diameter D is formed, and the line width of the line drawing 240 is actually nonuniform.
A more specific example is shown in FIGS. 14A to 14D and FIGS. 15A to 15C, which show line drawings 242 to 246 that are formed when two or more ink droplets are discharged with the following discharge conditions: an ink quantity of 2 pL per single discharge, a dot diameter D=30 μm when the dots are formed singly by the ink droplets, a pitch of P=10 μm between the dots, and a droplet discharge frequency that is 10% of the ink permeation time.
It should be noted that in FIGS. 14A to 14D and FIGS. 15A to 15D, the same key symbols are assigned to the same or similar portions as FIGS. 13A and 13B, and a description thereof is omitted.
FIG. 14A shows the case in which a single ink droplet is discharged to form a single dot 202. When a single ink droplet is discharged, a single dot is formed with a diameter of 30 μm.
FIG. 14B shows a line drawing 242 formed when five ink droplets have been successively discharged under the above-described conditions. It should be noted that the advancing direction of the droplet discharges is indicated by the arrow K and is the left-to-right direction in FIG. 14B.
When droplets are discharged so that the next ink droplet lands before the previously landed ink droplet completes permeation, as shown in FIG. 14B, the subsequently landed ink droplet is drawn to the previously landed ink droplet.
This situation is essentially the same as when an ink amount that is greater the prescribed 2 pL lands at the landing position of the previous ink droplet, and an ink amount that is less than the prescribed 2 pL lands at the landing position of the following ink droplet. The shape formed by these droplets is different from the shape formed by two dots with the same dot diameter that should be formed, a dot is formed with a diameter that is greater than the dot that was intended to be formed at the landing position of the previously landed ink droplet, a dot is also formed with a diameter that is less than the dot that was intended to be formed at the landing position of the following landed ink droplet, and the resulting shape is essentially the same as the shape of the two joined together.
In a similar manner, the third ink droplet is drawn to the ink droplet in which the first and second ink droplets have been joined together, and the phenomenon whereby the most recently landed ink droplet is drawn to the previously landed ink droplets. It should be noted that ratio (variation amount of ink) of recently landed ink droplets drawn to the previously landed ink droplets decreases as droplet discharge advances.
The ultimate shape of the line drawing 242 formed in this manner after the ink has completed permeation results in a state in which the width h1 (=40 μm) on first-droplet side (writing start side, left edge in FIG. 14B) is greater than the width hn (h5=20 μm in FIG. 14B) on the last-droplet side (writing end side). The density on the writing-start side is higher than the density on the writing-end side. This is due to the fact that the amount of droplet discharge on the writing start side (left side in FIG. 14B) is greater than that on the writing end side (right side in FIG. 14B) because the second droplet and later droplets are drawn to the liquid on the writing start side that landed previously.
It should be noted that the line width h1 of the edge of the writing start side is 40 μm, which is greater than the intended line width of 30 μm, and the line width h5 of the edge of the writing end side is 20 μm, which is less than the intended line width of 30 μm. The line width of the line drawing 242 gradually decreases from the writing start side toward the writing end side.
FIG. 14C shows a line drawing 244 formed when the 10 ink droplets are successively discharged under the above-described conditions, and FIG. 14D shows a line drawing 246 formed when the 60 ink droplets are successively discharged under the above-described conditions.
In the line drawing 244 formed with 10 ink droplets, the width h1 on the writing start side is 45 μm, and the width hn (h10 in FIG. 14C) on the writing end side is 20 μm, and since the width of the line drawing 244 gradually changes, the slope (when the width changes) is substantially fixed, as shown in FIG. 14C.
In the line drawing 246 formed with 60 ink droplets, the width h1 on the writing start side is 50 μm, and the width hn (h60 in FIG. 14D) on the writing end side is 15 μm, as shown in FIG. 14D. However, the slope of the width of the line drawing 246 shown in FIG. 14D is different depending on the writing start area 260, intermediate area 262, and writing end area 264.
In other words, in the writing start area 260, the width of the line drawing 246 gradually decreases from h1 (h1=50 μm in FIG. 14D) to hi (hi=30 μm in FIG. 14D); in the intermediate area 262, the width of the line drawing 246 does not vary from hi; and in the writing end area 264, the width of the line drawing 246 gradually decreases from hi to hn (h60=15 μm in FIG. 14C).
FIGS. 15A to 15D show the three-dimensional shape (cross-sectional shape) of the ink that forms the line drawings 242, 244, and 246 shown in FIGS. 14A to 14D.
In the line drawings 242 and 244 formed with about five to 10 droplets, as shown in FIGS. 15B and 15C, the height of the ink droplets gradually changes from the writing start side toward the writing end side.
Conversely, in the line drawing 246 shown in FIG. 15D, the height of the ink droplets gradually vary from the writing start side toward the writing end side in the writing start area 260 and writing end area 264, and the height of the ink droplets tend to remain unvaried in the intermediate area 262.
Therefore, when the ink droplet subsequently discharged before the previously landed ink droplet completes permeation lands so as to overlap the previously landed ink droplet, a phenomenon occurs whereby the width of the formed line drawing becomes nonuniform, and the quality of the resulting image is reduced due to landing interference that occurs on the recording paper, as shown in FIGS. 14A to 14D and FIGS. 15A to 15D. The phenomenon is difficult to observe as a difference in line thickness, but differences in line density are easily observed, and the phenomenon is particularly easily observed when a plurality of line drawings are formed next to each other.
It should be noted that the numerical values shown in FIGS. 13A and 13B as well as FIGS. 14A to 14D are merely examples, and the values may vary depending on the type of ink, type of recording paper (recording medium), and combinations thereof.
In the recording method and apparatus thereof cited in Japanese Patent Application Publication No. 9-272226, the configuration outputs the original recording data and interpolation data during separate head scans and carries out recording in order to prevent mutual interference of adjacent dots, and ink bleeding and mixing between the inks are prevented.
However, the discharge cycle cannot be reduced with control in which the subsequent ink lands after the previously landed ink droplet completes permeation. Also, since the permeation time of the ink (permeation velocity) varies depending on the combination of ink and recording medium, control must be carried out so as to vary the discharge cycle in accordance with the type of ink and recording medium that is to be used, and the control load of the apparatus is increased.
In the recording method and apparatus thereof cited in Japanese Patent Application Publication No. 9-272226, the original recording data scan and the interpolation data scan must be separately carried out when dots are formed with high density, a reduction in productivity is unavoidable, and it is difficult to achieve both higher image quality and higher printing speed.